1. Field of the Invention
The principles of the present invention relate to liquid crystal display devices. More particularly, the principles of the present invention relate to a color filter-on-thin film transistor (COT) substrate incorporating a color filter and a method of fabricating the same, wherein such a COT substrate prevents TFTs from malfunctioning due to exposure to external light as well as reducing reflections of external light by signal lines without using a black matrix.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices control light transmissivity characteristics of liquid crystal material with electric fields to display pictures. Thus, LCD devices typically include an LCD panel having liquid crystal cells arranged in a matrix pattern and a drive circuit for driving the LCD panel.
LCD panels generally include a TFT substrate and a color filter substrate facing each other, a liquid crystal layer injected between the two substrates, and a spacer to maintain the distance between the two substrates (i.e., a cell gap). The TFT substrate generally includes a pixel electrode formed within each liquid crystal cell, wherein the liquid crystal cells are defined by crossings of gate and data lines; a TFT connected between the gate line, the data line, and the pixel electrode; a plurality of insulating films; and an alignment film formed over the resulting structure. The color filter substrate generally includes a color filter layer having color filters individually aligned with the liquid crystal cells of the TFT substrate; a black matrix film to visually isolate adjacent color filters and to reflect external light; a common electrode to supply a reference voltage to the liquid crystal layer; and an alignment film formed over the resulting structure.
After providing the TFT and color filter substrates, formation of the LCD panel can be completed in one of two ways: the TFT substrate and the color filter substrate is bonded together, liquid crystal material is injected into the cell gap, and the cell gap sealed; or liquid crystal material is dispensed on either the TFT or color filter substrate and the two substrates are then bonded together. In either process, the two substrates are aligned before they are bonded together so that each color filter of the color filter substrate is aligned with an individual pixel electrode of the thin film transistor substrate. Accordingly, if the two substrates are not aligned correctly, the LCD panel exhibits a light-leakage defect. To minimize the effects of the light-leakage defect, the width of the black matrix film can be increased. This solution, however, is undesirable because use of such a black matrix film results in an LCD panel having a deteriorated aperture ratio. Accordingly, a color filter-on-TFT (COT) substrate has been developed, wherein the aforementioned color filter layer is incorporated within the TFT substrate.
FIG. 1 illustrates a plan view of a related art COT substrate. FIG. 2 illustrates a sectional view of the COT substrate shown in FIG. 1, taken along the line II-II′.
Referring to FIGS. 1 and 2, a related art COT substrate includes a substrate 1, a TFT array formed on the substrate 1, a color filter layer on the TFT array, a black matrix film 32 formed over the TFT array and color filter layer, an overcoat layer 52 on the color filter layer and the black matrix film 32, and pixel electrodes 22 on the overcoat layer 52. The TFT array generally includes gate lines 2, data lines 4, TFTs 30 and a protective film 18 while the color filter layer typically includes red (R), green (G), and blue (B) color filters 34. Moreover, each of the pixel electrodes 22 overlap with individual ones of the R, G, and B color filters 34 and are separated therefrom by the overcoat layer 52.
The gate and data lines 2 and 4, respectively, cross each other over the substrate 1 and are separated from each other by a gate insulating film 12. Accordingly, pixel areas are defined by the crossings of the gate and data lines 2 and 4. Each TFT 30 includes a gate electrode 6 connected to a corresponding gate line 2, a source electrode 8 connected to a corresponding data line 4, a drain electrode 10 opposing the source electrode 8, an active layer 14 overlapping the gate electrode 6 and separated therefrom by the gate insulating film 12 to form a channel between the source and drain electrodes 8 and 10, and an ohmic contact layer 16 to reduce a contact resistance between the active layer 14 and the source and drain electrodes 8 and 10. The protective film 18 is formed over the gate insulating film 12 and covers the TFTs 30 and the data lines 4.
The R, G, and B color filters 34 are formed on the protective film 18 and are confined to the pixel areas defined by the crossings of the gate and data lines 2 and 4 (i.e., at most, the R, G, and B color filters 34 partially overlap adjacent ones of gate and data lines 2 and 4). Therefore, the COT substrate shown in FIGS. 1 and 2 includes discretely arranged R, G, and B color filters 34 that are completely isolated from each other.
The black matrix film 32 is formed on the protective film 18, and overlaps the gate and data lines 2 and 4, the TFTs 30, and peripheral portions of R, G, and B color filters 34. Formed as described above, the black matrix film 32 prevents the generation of light-leakage between adjacent the color filters 34, reflects external light, and prevents photo-induced leakage current, generated when the channel of the TFT 30 is exposed to external light.
The overcoat layer 52 is provided as an organic insulating material and is formed on the R, G, and B color filters 34 and on the black matrix film 32 to compensate for the step difference that exists between the R, G, and B color filters 34 and the black matrix film 32, thereby forming an even surface and preventing impurities from the color filters 34 and the black matrix 32 to contaminate a subsequently provided liquid crystal layer.
Each pixel electrode 22 is formed on the overcoat layer 52 and is aligned with a pixel area so as to overlap with an individual color filter 34. Further, each pixel electrode 22 is connected to a corresponding drain electrode 10 via a contact hole 20 formed through the overcoat layer 52, a corresponding color filter 34, and the protective film 18.
Constructed as described above, each pixel electrode 22 must contact a corresponding drain electrode 10 via a contact hole 20 that, undesirably, is formed through the entire thickness of a corresponding color filter 34. When a contact hole is formed through the entire thickness of a color filter, a potentially unattractive light transmission pattern can result. Moreover, it is desirable but difficult for the contact hole 20 to be sufficiently narrow through the entire thickness of the color filter 34. As a result, it is desirable but difficult for the contact hole 20 to have a high aspect-ratio (i.e., a large contact hole depth compared to the contact hole width). To overcome this difficulty, it has been proposed to form the contact hole 20 in a region that overlaps the gate line 2 where no color filter 34 is formed but where the protective film 18 and the overcoat layer 52 are formed. Accordingly, the contact hole 20 can be formed in two mask processes wherein the protective film 18 and the overcoat layer 52 are separately patterned. However, because the black matrix film 32 overlaps the gate line 2 and peripheral portions of the color filters 34, the contact hole 20 must also be formed through the black matrix film 18. As a result, an additional mask process must be used to form the contact hole 20, wherein the black matrix film 32 is additionally patterned. Thus the number of processes required to form the contact hole 20 in a region overlapping the gate line 2, where no color filter 34 is formed, undesirably increases and results in increased manufacturing costs and decreased productivity.